1. Field of the Invention
The present invention relates to a scanning probe microscope, and more particularly, relates to a scanning probe microscope making it possible to shorten a contact time for a probe tip being contact with a sample surface and thereby perform measurement with smooth movement of the probe tip on the sample surface.
2. Description of the Related Art
A scanning probe microscope (SPM) is a measurement equipment capable of measuring objective samples of an atomic size level in its resolution. The scanning probe microscope is being used in various kinds of wide fields, such as measurement of the surface shape of substances or materials and measurement of the surface shape of semiconductor-chips including LSI. The scanning probe microscope is provided with a cantilever having a probe tip at its pointed end. The scanning probe microscope measures the sample by detecting a physical amount generated between the probe tip and the sample when making the probe tip approached to the sample to be measured with a required distance. There are various kinds of the scanning probe microscopes responding to the physical amounts used for the detection, such as a scanning tunneling microscope (STM), an atomic force microscope (AFM) and a magnetic force microscope (MFM). For this reason, the applicable field for the scanning probe microscope is being enlarged presently.
Among the above-mentioned microscopes, the atomic force microscope is suitable when detecting the surface shape of the sample with high resolution, and it actually becomes useful in the fields of the semiconductor devices, the optical disks and so on. Hereinafter, the atomic force microscope will be explained.
The principles of the measurement based on the atomic force microscope are roughly divided into xe2x80x9ccontact modexe2x80x9d and xe2x80x9cnon-contact modexe2x80x9d in response to the relationship produced between the probe tip and the sample surface. In the current stage, the measurement of the contact mode is mainly used in the technical field of industrial surface form measurement, because the measurement of the non-contact mode is slow in a measuring speed.
The outline of fundamental structure of the atomic force microscope performing the surface shape measurement in the contact mode is as follows.
In the atomic force microscope, a coarse movement mechanism section is fixed to a fixing part such as a support frame, and a fine movement mechanism section is attached to the lower part of the coarse movement mechanism section, and a cantilever is further attached to the lower end of the fine movement mechanism section. A probe tip is formed at the tip of the cantilever. The probe tip is directed to the surface of the sample placed at a lower spot in the state of approaching it to the sample. The above-mentioned coarse movement mechanism section is a means for approaching the probe tip to the surface of the sample in the height direction (Z direction) with a comparatively large distance, and it is used for an early approach movement of the probe tip. The fine movement mechanism section is a means for moving the probe tip to three-dimensional directions (each axis direction of X-axis, Y-axis and Z-axis intersecting perpendicularly mutually) in a comparatively fine distance. The fine movement mechanism section is comprised of a XY fine movement section for moving the probe tip along the sample surface directions (XY directions) as a scanning movement, and a Z fine movement section for moving the probe tip to the height direction. A control section controls operations of the coarse and fine movement mechanism sections. The cantilever is moved downward by the operations of the coarse and fine movement mechanism sections. When the probe tip approaches the sample surface sufficiently, the atomic force given from the sample surface to the probe tip causes the cantilever to be bent to make the cantilever deformation. Displacement detection means comprised of a laser light source and an optical detector detects the deformation of the cantilever. The laser light emitted from the laser light source is irradiated onto the back of the cantilever, and then the laser light reflected on the back of the cantilever enters the light-receiving surface of the optical detector. In accordance with the arrangement of the displacement detection means, when the deformation arises in the cantilever, the displacement of the probe tip in the Z direction can be detected, since the laser light incidence position on the light-receiving surface of the optical detector changes. The information on the position of the probe tip in the height direction, which is detected by the optical detector, is compared with a standard position (target standard value) set up beforehand, and the difference obtained by the above comparison is inputted into the above-mentioned control section. On the basis of the information on the difference, the control section gives a signal used for controlling the operation of Z fine movement section to the Z fine movement section so that the height of the probe tip to the sample surface (the difference between the sample and the probe tip) may be consistent with the standard position.
The configuration mentioned above makes the probe tip of the cantilever scan the shape of the sample surface to follow it, detecting the atomic force produced between the sample surface and the probe tip and controlling the distance between the sample surface and the probe tip to be constant (target standard value). In this measurement operation, the control section is usually configured by a proportion and integration control (PI control). In order to keep the distance between the sample surface and the probe tip constant, the atomic force between the sample surface and the probe tip can be kept contact.
When measuring the shape of the sample surface by following it based on the scanning operation while the distance between the sample and the probe tip is kept constant, as mentioned above, the contact mode is used. There are some modes in the contact mode, and they are shown in FIGS. 4A, 4B and 4C. FIG. 4A shows a static contact mode and FIGS. 4B and 4C show dynamic contact modes. In FIGS. 4A-4C, a reference number 101 designates the pointed end of the probe tip, and 102 the sample.
The static contact mode measurement is a most general method. In this measurement, the probe tip is continuously moved between each two of measuring points {circle around (1)}-{circle around (5)} along the surface of the sample 102 as shown by an arrow 103. The measurement of this method makes it possible to perform a high-speed measurement in a viewpoint of time and space because it is performed with the continuous operation.
The Dynamic contact mode is arranged so that the probe tip 101 may once be separated from the surface of the sample 102 with the advance of the scanning movement. As to the dynamic contact mode, FIG. 4B shows the method of contacting the probe tip 101 onto the sample surface only at the measuring points {circle around (1)}-{circle around (5)} as shown by an arrow 104, and FIG. 4C shows the method of repeating the contact and separation by making the probe tip 101 or the cantilever resonate in the Z direction as usually shown by arrow 105 using a sine wave etc. (several tens to several hundreds kHz). In FIG. 4C, the movements of contact and separation are also repeated in spots other than the measuring point {circle around (1)}-{circle around (5)}.
The measurement method by the above-mentioned static contact mode is unsuitable for samples which have steep level differences or generate large frictional forces, because the probe tip receives the force in the scanning direction or the frictional force with the advance of the scanning movement. Furthermore, if a large lateral force is operated to the probe tip, the sample surface is damaged, and therefore it is also unsuitable for the measurement of soft samples.
The above-mentioned dynamic contact mode shown in FIGS. 4B and 4C can solve the problem about the lateral force in the static contact mode, and has the advantage of being suitable for the measurement of the sample with the steep level differences or large frictional forces. Furthermore, since the contact time for the probe tip onto the sample surface is short in case of the dynamic contact mode shown in FIG. 4B, it also has the advantage that there is little wear at the pointed end of the probe tip. However, since discontinuous operations are repeated in the aspect of time and space in the case shown by FIG. 4C, the case has the problem that it requires a lot of time for the operation and the measurement speed becomes slow. On the other hand, the dynamic contact mode of FIG. 4C has the property of continuity like the static contact mode and is also suitable for the high-speed measurement because of repeating the contact and separation movements by high frequency. However, the case shown by FIG. 4C has the problem of receiving the force of the lateral direction not a little, because of increasing the contact time and repeating the contact operations during the scanning movement, as compared with the case shown by FIG. 4B. The measurement method of FIG. 4C has a disadvantage in a viewpoint of the wear at the pointed end of the probe tip.
An object of the present invention is to solve the above-mentioned problems, and is to provide a scanning probe microscope capable of performing a high-speed measurement without receiving the lateral force due to the scanning movement when moving the probe tip along the sample surface, and reducing the wear of the probe tip.
The scanning probe microscope of the present invention has the following configuration in order to attain the above-mentioned object.
The first scanning probe microscope is provided with a cantilever with a probe tip facing a sample, a Z fine movement section for changing a distance between the sample and the probe tip, a XY scanning control section for providing relative displacement toward a sample surface between the sample and the probe tip, a displacement detecting section for detecting displacement arising in the cantilever, and a Z direction control section. The cantilever is preferably attached to the Z fine movement section. The displacement detecting section is preferably an optical lever type displacement detection mechanism configured to use a laser light, which contains a laser generator and an optical detector. In accordance with the above configuration, when the deformation arises in the cantilever due to a physical amount between the probe tip and the sample, the displacement detecting section detects the displacement of the cantilever due to the deformation thereof, and the Z direction control section controls so as to keep the displacement of the cantilever a predetermined constant value and thereby the physical amount on the surface of the sample is measured. Further the scanning probe microscope comprises a two frequency signals generating section for providing signals used to cause the probe tip to be moved in height direction by at least two frequencies to the Z fine movement section as its characteristic part. By this two frequency signals generating section, the probe tip is moved in the height direction using the at least two frequencies, and the physical amount generated between the probe tip and the sample surface is detected when the probe tip approaches the sample based on the second frequency.
According to the present invention with the above-mentioned structure, in the measurement operation based on the dynamic contact mode in which the contact and separation movements are repeated, the repeated movement is performed by using the cyclic signals of the two frequencies. In this case, it is not limited to the two cyclic signals and two or more cyclic signals may also be used. Using the two cyclic signals of the two frequencies can simultaneously realize the reductions of high-speed measurement and contact time because of the Z direction movement of the low frequency proportional to the cycle of the measuring points, and repeating the contact and separation movement by the high frequency.
The second scanning probe microscope has, in the above configuration, the feature that each movement based on the two frequencies is a sine wave movement. In accordance with this feature the high-speed measurement can be performed continuously in points of time and space because both of the two cyclic signals give smooth sine wave movements.
The third scanning probe microscope has, in the above configuration, the feature that concerning the movement based on the two frequencies, the movement due to the first frequency is a sine wave movement and the movement due to the second frequency is a trigger pulse movement.
According to the present invention, as mentioned above, in the measurement of the sample surface by the scanning probe microscope, when moving the probe tip along the sample surface with the required distance concerning the measurement area of the surface, the movement of the probe tip is made by the oscillations using the two frequency signals of high and low frequencies and thereby the contact time of the probe tip on the sample surface during the scanning operation is shortened. Thereby the lateral force the probe tip receives when it moves can be reduced, the wear of the pointed end of the probe tip is prevented, and the high-speed measurement can be performed.